A high-pressure discharge lamp, for example, a metal halide lamp, which substantially excludes mercury therefrom, is disclosed in Japanese laid-open patent JP11-238488A (hereinafter referred to as prior art I) etc. In the metal halide lamp disclosed in the prior art I, it is filled with two types of metal halides, i.e., a primary metal halide having relatively high vapor pressure and capable of mainly emitting light in visible range and an accessory halide hardly emitting light in the visible range in compared to the primary metal halide but contributing to fix lamp voltage, in place of mercury.
In the prior art I, as a first practical example, a metal halide lamp for liquid crystal projectors designed to have 4 mm inter-electrode distance and to operate at 150 W input power is described. In this first practical example, iodination dysprosium (DyI3) by 1 mg and iodination neodymium (NdI3) also by 1 mg are filled as a principal metal halide, respectively, and Argon (Ar) by 500 Torr is filled as rare gas. In this first practical example, when zinc iodide (ZnI2) by 8 mg is filled as an accessory halide, lamp voltage is 73V, luminosity is 68 lm/W, and color temperature is 9160K.
Further, in the prior art I, as an eighth practical example, a metal halide lamp designed to have 30 mm inter-electrode distance and to operate at 2 KW input power is described. In this first practical example, 4 mg dysprosium bromide (DyBr3), 4 mg holmium bromide (HoBr3), and 4 mg thulium bromide (TmBr3) are filled as the principal metal halide, respectively, and 100 Torr Argon (Ar) is filled as rare gas. In this eighth practical example, when 30 mg zinc iodide (ZnI2) is filled as the accessory halide, lamp voltage is 112V, luminosity is 92 lm/W, color temperature is 5340K, and a average color rendition evaluation account is Ra73.
On the other hand, a high-pressure discharge lamp improved luminosity, light color, and life duration, but containing mercury as buffer gas is disclosed in Japanese laid-open patent JP2004-349242A (hereinafter referred to as prior art II). By carrying out the mass percentage of the sodium halide, the thallium halide, the indium halide, and the thulium halide into a prescribed range, respectively
The metal halide lamp disclosed in the prior art I has acquired decent electrical property and luminescent property close to those of the conventional metal halide lamp using mercury, without using mercury of high environmental burden. However, an appearance of mercury-free metal halide lamp having luminosity sufficiently higher than conventional metal halide lamp is expected.
It is known to use Sodium (Na) as substance for emitting white light high-efficiently together with, for example, Scandium (Sc) and a rare earth metal. However, the D line of Sodium (Na) is an emission spectrum of 589 nm wavelength, and is separated from 555 nm, which is separated from 555 nm, i.e., peak of luminosity curve. Therefore, it is impossible to acquire sufficiently high luminosity with only Sodium (Na). So, in order to further advance efficiency, it is necessary to raise a temperature of the coldest part.
However, since there are various restrictions, such as the heat-resisting property of airtight envelope, the reactivity of Sodium (Na), etc. which constitute an arc tube, it is difficult to dramatically improve luminosity. In addition, in a mercury-free high-pressure discharge lamp, although Sodium contributes to improve luminosity, Sodium (Na) has a, fault which makes the inter-electrode potential gradient gentle, and results in reduction of lamp voltage. In order to supply a desired lamp electric power, it is necessary to make lamp current increase, since lamp voltage comes down in the case of that a discharge medium includes large quantity of Sodium, as mentioned above. For making lamp current increase, it is necessary to thicken the diameter of rod electrode. However, if a rod electrode is made thick, not only the design of the electrode itself and an airtight envelope becomes difficult, but also the design of stabilizer will also become difficult.
By the way, although the metal halide lamp of the prior art I is able to achieve electrical property and luminosity almost equivalent to those of conventional metal halide lamp using mercury, it is inferior in luminescence efficiency to the metal halide lamp using mercury.
On the other hand, in the mercury-free discharge lamp of the prior art II, since it is premised on using mercury as buffer gas, such a favorable luminosity as described are not obtained.
In a mercury-free high-pressure discharge lamp, thulium halide is suitable for emission medium. This is because that Thulium has an innumerable emission spectrum around the peak of luminosity curve, and that proper amounts of emission spectrum exist in short wavelength range from the peak of luminosity curve. However, the melting point of the iodination thulium often used in a mercury-free high-pressure discharge lamp is as high as 1030 degrees C. Therefore, in order to ionize Thulium and make it produce luminescence, it is necessary to raise arc tube temperature in matching with the above-mentioned melting point of iodination thulium.
However, if arc tube temperature is raised as mentioned above, the life of a mercury-free high-pressure discharge lamp will become shortened. Further, although iodination thulium can be pelletized by mixing with other halide substances, the iodination thulium fails to be pelletized alone, and only turns out powder. Therefore, it is difficult to include required amount of iodination thulium in the light-transmissive airtight envelope of a mercury-free high-pressure discharge lamp
An object of the present invention is to provide mercury-free high-pressure discharge lamp in easy to manufacture, excellent in life property, luminosity and electrical property and luminaire using this mercury-free high-pressure discharge lamp.